This project is part of a research program that seeks to understand molecular mechanisms of normal and abnormal limb pattern formation through the characterization of novel genes defined by mutations and polymorphisms in the mouse. Outgrowth and proximodistal patterning of the vertebrate embryonic limb require signaling by the apical ectodermal ridge (AER) to the progress zone (PZ), which proliferates in response and lays down the cells of the presumptive limb in a proximal to distal progression. The signaling loop between AER and PZ that is essential for sustaining outgrowth and patterning in the limb is disrupted in the antimorphic mouse mutation Dactylaplasia (Dac). Dac was cloned by position and found to encode a pioneer gene whose sequence reveals nothing about its function. The suppressor allele of a strain polymorphism, Modifier of Dactylaplasia (Mdac), has no phenotype on its own but dominantly suppresses Dac with 100 percent penetrance. The proposed research has three aims: (1) to understand the developmental function of the Dac gene by (a) generating and characterizing a targeted null allele and (b) determining the mechanism by which the two spontaneous mutations Dac1J and Dac2J exert their dominant effect; (2) to generate reagents (antibodies and a full length cDNA) that will allow future characterization of the biochemical and cell biological functions of the Dac gene; and (3) to clone Mdac by position. The experiments are designed to use the Dac/Mdac system as a novel entry point to investigate epithelial-mesenchymal interactions in growth and developmental patterning. Because Dac is likely to be the mouse ortholog of the human Split Hand/Foot Malformation 3 gene, this project may also bear direct relevance to understanding and diagnosis of congenital limb diseases.